1. Field of the Invention
The present invention relates to a process and installation for coating by electrophoresis the surface of a substrate immersed in an electrophoretic bath, and more particularly to an process and installation for coating where the bath in the vicinity of the surface is subjected to vibrational movements, particularly at sound or ultrasound frequencies.
2. Discussion of the Background
Painting by means of electrophoresis is mainly used for parts of an automobile body. The electrophoretic bath is generally comprised of an aqueous solution of a film-forming polymer material; polyepoxide type resins are widely used. An electrophoretic electric current is used to take the particles of the emulsion toward the part to be painted where they will comprise the paint layer; the electrical resistance between the part to be painted and the counter electrode increases with the thickness of the deposit.
Surface defects may be generated during this process. The surface defects of the paint layer have the form of craters which, on sheets of steel, are sites where corrosion tends to begin; in addition, in spite of the three additional layers of paint (respectively called xe2x80x9csealer,xe2x80x9d xe2x80x9cbasexe2x80x9d and xe2x80x9cvarnishxe2x80x9d) which one subjects the visible parts of the vehicle body to above the cataphoresis layer, the craters remain visible and greatly degrade the appearance of these parts. These craters are present in the form of small cone-shaped holes which open onto the surface of the cataphoretic layer; they have a diameter generally between 100 and 500 micrometers at the base, between 5 and 20 micrometers at the top. These so-called xe2x80x9ccraterizationxe2x80x9d defects result from the formation of a gas, particularly hydrogen, in the vicinity of the surface area of the part during coating.
An automobile body painting unit in the traditional manner includes a container of paint and a conveyor unit for immersing the part in the bath, moving it along the bath and extracting it from the bath, as described in JP 87-268321 A, for example. The length of the container and the movement speed of the part in the container are adjusted to the thickness of the paint layer to be deposited, depending upon the paint depositing rate. The rate of depositing is proportional to the electric field in the vicinity of the part to be painted; that is, the potential difference applied between the electrode and the back electrode; with constant polarization, this speed decreases as a function of the time until it is nearly canceled when the thickness of the deposited paint layer offers a considerable electrical resistance to passage of the electrophoretic current. The part extracted from the bath is dried in order to ensure baking of the coating; for polyepoxide-type resins, the drying process lasts about 20 minutes at approximately 180xc2x0 C.
As described in JP 87-268321A, when one applies a paint coating in this manner onto sheets of steel coated with zinc or a zinc alloy, especially sheets of alloy galvanized steel, one will observe surface defects (xe2x80x9cpinhole gasesxe2x80x9d) on the layer of paint, which result from the formation of gas bubbles on the surface to be painted during electrical deposition. In order to prevent the formation of these defects, JP 87-268321A proposes that one can subject the electrophoretic bath to vibrational movements at ultrasound frequencies during the passage of the electrophoretic current.
In order to produce vibrations in the bath, one immerses ultrasound-emitting generators in the bath along the movement path of the part, on either side of the part; these ultrasound emitters are distributed on either side of the movement path along two longitudinal walls of the paint container (reference numeral 7 in FIGS. 1 and 2 of JP 87-268329A) and are connected to an adjustable power supply device. This ultrasound electrodeposition process is expensive because it requires the installation of many emitters along the movement path of the parts.
An object of the invention is to provide a process and system for coating a surface by electrophoresis which are more economical.
Another object of the invention is to provide a process and system for coating by electrophoresis a surface with no or fewer resulting defects.
A further object of the invention is to provide a process and system for coating a surface where deposition rates may be improved.
These and other objects are achieved by a coating process by electrophoresis of a surface of a substrate immersed in an electrophoresis bath, comprising steps of applying an electrical current to the surface, during applying the current, subjecting one of the bath and the sample to vibrational movements to generate vaporous cavitations in a vicinity of the surface, and applying the vibrational movements for a period substantially less than a time period over which the current is applied. Generating the vibrational movements may be performed only during at least one of an initial phase at a beginning of application of the current and a second phase at an end of application of the current. The initial phase may begin approximately at the onset of the application of the current and ends before a time corresponding to half of a duration of application of the current, and the second phase may begin after this time and ends approximately at an end of the application of the current.
The end of the initial phase may occur approximately at a moment corresponding to an inflection point of a characteristic, as a function of time, of electrical resistance measured between the surface area and a counter-electrode under the same conditions of the coating but in the absence of the the vibrational movements.
The vibrational movements may be generated in the initial phase for no more than one fourth of a duration of the application of the current. The vibrational movements may be generated only in the initial phase, or only in the second phase.
During the initial phase, a current may be applied to produce a polarization voltage greater than a crater forming voltage of the surface. The current may be applied such that a duration of a rise of the polarization voltage rise up to a predetermined value greater than the crater forming voltage is less than 1 second.
The process may also include steps of determining an inflection point in a characteristic of electrical resistance between the surface and a counter electrode as a function of time determined in absence of the vibrational movements, and stopping the generation of the vibrational movements at a time approximately corresponding to the inflection point.
The vibrational movements may be generated in the bath using one of sound and ultrasound waves, or they may be generated by vibrating the substrate at one of sound and ultrasound frequencies. The vibrational movements may be generated only in the vicinity of predetermined zones of the surface area.
The process and system may be applied to coating a substrate made of alloy galvanized steel.
The process may include steps of immersing the substrate in the bath, conveying the substrate through the bath, and extracting the substrate from the bath. In this case the vibrational movements may be generated only during at least one of an initial phase including the immersing step at a beginning of application of the current and a second phase including the extracting step at an end of application of the current or generated in the initial phase for no more than one fourth of a duration of the immersing, conveying and extracting steps. The vibrational movements may be generated only in the initial phase or second phase, or only in the vicinity of predetermined zones of surface during the second phase. The end of the initial phase may occur approximately at a moment corresponding to an inflection point of a characteristic, as a function of time, of electrical resistance measured between the surface area and a counter-electrode under the same conditions but in the absence of the the vibrational movements.
The process according to the invention may also comprise immersing a surface in an electrophoresis bath, extracting the surface from the bath, and applying vibrational movements to one of the surface and the bath to generate vaporous cavitations in the vicinity of the surface at only at least one of an immersion point and an extraction point. The vibrational movements may be applied only the immersion point or only at the extraction point, or only to predetermined portions of the surface. A current may be applied to the surface using a counter-electrode and applying the vibrational movements may be performed during application of the current up to approximately a moment corresponding to an inflection point of a characteristic, as a function of time, of electrical resistance measured between surface and a counter-electrode in the absence of the vibrational movements.
These and other objects may be achieved by a system for coating of a surface of a part by electrophoresis comprising a container holding an electrophoresis bath and a vibrational generator to apply vibrational movements to the bath to generate vaporous cavitations in the vicinity of the surface disposed at only at least one of an immersion point and an extraction point in the container for the part. The system may include a device to immerse the surface area in the bath, to remove the surface area from the bath, and to convey the part from the immersion point to the extraction point. The system may also include a counter electrode for applying an electrical current to surface, where the vibrational generator applies the vibrational movements during application of the current.
The vibrational generator may be disposed to generate the vibrational movements only in an immersion zone ending at approximately at a location in the container corresponding to an inflection point of the curve R(t) of evolution, as a function of time, of electrical resistance measured between the surface and the counter-electrode in the absence of the vibrational movements. It may also be positioned to generate the vibrational movements substantially only in an immersion zone in a direction of movement having a length no more than one fourth of a length of the container. The vibrational generator may be one of a sound generator and an ultrasound generator.